U.S. Pat. No. 4,188,661, "Direct Drive Ballast With Starting Circuit" by Bruce L. Bower and Raymond H. Kohler, dated Feb. 12, 1980 and assigned to the assignee of the present invention and hereby incorporated by reference, describes an electronic ballast circuit for driving a pair of fluorescent lamps. Central to the operation of that circuit is a high frequency (20 to 30 KHz) inverter comprising two transistors connected in series and operating in a push-pull mode. The inverter drives, via an output transformer, the cathode filaments of the lamps. The output transformer comprises a series-resonant primary winding coupled to the inverter output. The secondary of the output transformer includes three filament windings, two for separately supplying current to one filament of each of the lamps. The third filament winding supplies current to the remaining two, parallel-connected filaments. Also included on the secondary of the output transformer is a lamp drive winding connected to a pair of bias windings. The bias windings are oppositely poled and connected in series between the first and second filament windings. These windings are arranged so as to establish a voltage differential across the cathodes of the respective lamps sufficient to effect firing of the lamps.
The ballast circuit further includes an interstage transformer having three primary windings each coupled in a loop that includes at least one lamp filament and a lamp filament winding. The secondary of the interstage transformer includes a pair of oppositely-poled base drive windings coupled to the push-pull inputs of the inverter. Because the primary windings are coupled in a loop that includes the lamp filaments, they include voltages in the base-drive windings proportional to the sum of filament currents. Proper phasing of the secondary windings provides the positive feedback necessary to sustain inverter operation. (A modified feedback arrangement disclosing a single primary winding connected in a loop with the two parallel-connected filaments is disclosed in U.S. Pat. No. 4,127,893, "Tuned Oscillator Ballast Circuit With Transient Compensating Means" by Charles A. Goepel and assigned to the assignee of the present invention. See FIG. 2 of that patent).
U.S. Pat. No. 4,188,661 also discloses circuitry for enhancing the oscillator startup operation. Upon initial energization of the ballast circuit, a capacitor connected in parallel with one of the secondaries of the interstage transformer is slowly charged through a source of slowly developed DC voltage. When the charge across the capacitor reaches a given magnitude, a series connected diac is switched on thereby discharging the capacitor through a relatively low impedance and causing a transient across the secondary of the interstage transformer. This perturbation supplies base drive to at least one of the inverter transistors and assures oscillator startup. A voltage derived from the current in the primary of the output is rectified and applied to the diac in a manner that renders the diac nonconducting during steady state operaton of the ballast circuit.
While it cannot be gainsaid that the circuitry disclosed in the patent discussed above represents a substantial advance in the state of the art of ballast design, with regard to both the conventional electromagnetic and electronic types, the subject invention represents a further substantial advance in that art. In particular, the improved output transformer configuration disclosed herein allows independent selection of both the pre- and post-ignition lamp filament voltages. This feature is highly desirable because the differences in lamp filament pre- and post-ignition voltages impose constraints on the design of the ballast output configuration that render the maintenance of the pre-ignition voltage below filament arc-over voltage difficult to achieve.